squash: [wip]: gir xml schema
giscanner: remove C xml attribute formatter

Measuring the time it takes to build Gtk-3.0.gir
using a) the C version and b) the Python version
reveals there is no measurable difference to speak
of:

With C collect_attributes:
==========================

real	0m30.614s
real	0m32.255s
real	0m31.034s

With Python collect_attributes:
===============================

real	0m31.530s
real	0m31.549s
real	0m31.673s

Note that running g-ir-scanner under Python's
cProfile module does produce the 15 to 20%
difference mentioned in commit 98791e4955ea3955fec9c020bdfd3eee771ed241,
which is fully expected as cProfile does not
intrument the C version of the code but does
intrument the Python version, demonstrating
why one should not compare apples to oranges
when profiling  ;)